1. Field of the Invention
The invention relates to an apparatus for analysis of a sample by measurement of its ellipsometric parameters, to a procedure for ellipsometric analysis that can be carried out in particular with this apparatus, and to the application of this analyzer or of this procedure to the measurement of variation of thickness of thin films as a function of time, during growth or on the other hand etching of said films.
2. Discussion of Prior Art
The ellipsometric parameters of psi (ellipticity) and delta (phase difference) of an elliptical light vibration can be obtained using a plurality of existing apparatuses. Compared with the invention, they all suffer from the drawbacks of insufficient accuracy or of too slow operation. This latter disadvantage is particularly regrettable in the case of phenomena related to the evolution of thickness of thin films, where measurements every millisecond can be necessary.
For example, an apparatus is known in which a light beam produced by a laser passes through a plane-polarizing plate and a quarter-wave plate before reaching the sample to be analyzed, from which it is reflected before passing through an analyzer and being collected by a photodetector. The ellipsometric parameters are obtained by successive rotation of the polarizer and of the analyzer, when extinction of the light arriving at the photodetector occurs. This apparatus is not intended for rapid measurements, because the set of operations takes about twenty seconds.
A faster apparatus of the same family uses a rotating analyzer, the polarizer remaining immobile. The light collected by the photodetector varies according to a sine wave with two periods per rotation of the analyzer. The ellipsometric parametrers are determined by the ratio of the maximum and minimum amplitudes of the detected light, as well as by the phase of the sine wave.
The measurements with this apparatus are very sensitive to stray light rays that can no longer be recorded, since it is no longer required to bring about extinction of the beam. Another disadvantage is that the apparatus does not directly provide the value of the phase difference delta, but only its sine or cosine depending on whether a quarter-wave plate is used or not; uncertainties occur at the extremums of these circular functions.
Other known apparatuses, using a dichroic sample, for example, suffer from the same disadvantages.